Many materials produced in the form of a light, fluffy powder are pelletized prior to shipping in order to increase the bulk density of the material and improve the conveying and handling thereof. Examples of such materials include pigment grade silicas and carbon blacks which can be used, for example, as reinforcing agents for rubber, coloring or thickening agents for paints or inks, fillers for plastics, etc. Even though the pellets of such materials must necessarily have sufficient strength to withstand pulverization during the bulk shipment, conveying and handling thereof, it will nonetheless be appreciated that the pellets cannot be too resistive to a milling or grinding operation which is carried out for the purpose of redispersing the powder particles in the pellets so that they can be uniformly distributed within the medium in which they are to be used, e.g. a rubber, a paint vehicle, a plastic, etc. It is thus important that a test be available whereby the crushing strength of a pelleted material can be reliably determined in order to assure that produced pellets are strong enough to resist shipping and handling in bulk, but not so strong as to resist redispersion of the contained powder particles when the material is being distributed in such a medium.
One device for testing the crushing strength of pellets is disclosed in U.S. Pat. No. 3,331,241. In accordance with the description therein, pellets are glued to an elongated strand of tape at intervals, and the tape is then pulled through a gap between a force-measuring transducer and the end of a thrust-exerting rod which is continuously urged toward the foot of the transducer by means of a tensioned spring. The transducer delivers an output current at a voltage that varies in correspondence to the compressive force applied to the pellet for the purpose of crushing it. This voltage is measured with a voltmeter. A pellet, while attached to the tape, is thus dragged into a tapered gap between the force-exerting means and the foot of the transducer, and since this gap is smaller than the diameter of the pellet and since the forcing exerting means is continuously urged toward the foot of the transducer, the pellet is eventually crushed before it is pulled all the way through the gap.
Several disadvantages are associated with a pellet crushing strength tester of the type just described:
1. The resiliency of both the glue and the tape beneath each pellet can vary, hence interferring with accurate determination of the force actually required for crushing each pellet.
2. By pulling the tape into the tapered gap between the force-exerting means and the foot of the transducer, these members tend to be forced apart since the gap spacing is smaller than the diameter of the pellets, and this results in forces being exerted laterally against the pellet compression members. These laterally exerted forces detract from accurate determination of crushing strength since such forces can vary with differences in tape speed and the size and shape of the pellets.
3. In order to minimize error in measuring the crushing force, the tape and the glue for attaching the pellets thereto must be specially made up from carefully selected materials. In addition, a tedious procedure of attaching the pellets to the tape at spaced intervals must be followed, and still a second strand of tape must be used on the other side of the pellets to shield the working surfaces of the force-exerting means and the foot of the transducer from an accumulation of glue and powder.
4. The gap spacing between the force-exerting means and the foot of the transducer is critical in that it must be small enough to assure crushing of each pellet and yet large enough to assure transporting of pellets all the way through the gap by means of the tape.
It is therefore a principle object of the present invention to overcome the aforesaid disadvantages in a pellet crushing strength tester in order to simplify the testing procedure while obtaining reliable crushing strength test values. This and other objects and advantages of the invention will become apparent from the following description and the appended claims.